Skin, Hair, and Nails

With an average surface area of about 2 sq. m (2 1 / 2 sq. yd), the skin is one of the largest organs of the body. The skin forms a protective barrier between the harsh environment of the outside world and the body’s muscles, internal organs, blood vessels, and nerves. Hair and nails grow from the skin and provide extra protection. The appearance of the skin varies widely, not only changing with factors such as increasing age but also acting as a barometer of our fluctuating emotions and general health.

Surface of the skin

This magnified view of the hairless skin of the palm shows sweat pores arranged along ridges. The ridges help us to hold on to objects.

The skin is a living organ. The topmost layer of the epidermis, which forms the surface of the skin, is made up of dead cells, and a person sheds about 30,000 of these cells every minute. However, live skin cells are continually produced in the lower part of the epidermis to replace them. Below the epidermis lies the dermis, which contains blood vessels, nerve endings, and glands. A layer of fat lies under the dermis and acts as an insulator, shock absorber, and energy store.

Protecting and sensing

Even though most parts of it are less than 6 mm ( 1 / 4 in) thick, the skin is still a robust protective layer. The main component of its surface is a tough, fibrous protein called keratin. This substance can also be found in hair, which provides protection and warmth, and in nails, which cover the delicate ends of the fingers and toes. The skin forms a highly effective barrier against microorganisms and harmful substances, but it is most effective when the surface remains intact. Wounds may become infected and allow bacteria, some of which live on the skin’s surface, to enter the bloodstream. Sebum, an oily fluid formed in the sebaceous glands in the dermis, helps to keep the skin supple and repels water. It is because skin is waterproof that we do not soak up water like a sponge when we bathe.

Our sense of touch comes from receptors in the skin’s dermis that respond to pressure, vibrations, heat, cold, and pain. Every second, billions of signals from stimuli received all over the body are sent to the brain, where they are processed to create a sensory “image” and to warn of dangers, such as a hot cooker. Some sensory areas, such as the fingertips, have a high density of receptors. The skin also plays a major part in regulating body temperature and, when exposed to sunlight, produces vitamin D, which is essential for strong bones.

A responsive layer

Our skin responds to the life we lead. For example, the skin of a gardener’s hands becomes thickened, giving extra protection. The aging process, during which the skin becomes wrinkled and less elastic, can be accelerated by smoking or excessive exposure to sun. Skin can also change colour. In direct sunlight, the epidermis and dermis produce extra melanin, a pigment that filters harmful ultraviolet rays and causes the skin to darken. People originating in areas that have strong sunlight tend to have darker skin, which does not burn as easily as lighter skin. The skin of people with fair complexions has less melanin and is more susceptible to sunburn.

Structure: Parts of a Nail

Structure: Skin and Hair

Function: Growth and Repair

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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